In the related art, as this type of drilling tool, a drilling tool is known which includes: a casing pipe having a cylindrical shape; an inner bit which is inserted into the casing pipe in the direction of the axial line thereof and of which a distal end portion in the direction of the axial line protrudes from a distal end of the casing pipe; and a ring bit which has an annular shape, is disposed at a distal end portion of the casing pipe so as to be capable of rotating around the axial line relative to the casing pipe, surrounds the distal end portion of the inner bit, and is capable of engaging with the inner bit around the axial line and from the distal end side in the direction of the axial line (refer to, for example, PTLs 1 and 2 described below).
FIGS. 6 and 7 show a conventional drilling tool 100. In the drilling tool 100, a distal end portion of an inner bit 102 inserted into a casing pipe 101 protrudes from a distal end of the casing pipe 101. The inner bit 102 engages with a ring bit 103 rotatably disposed at the distal end of the casing pipe 101 so as to be rotatable integrally with the ring bit 103. Further, the ring bit 103 can engage with the inner bit 102 from the distal end side in the direction of the axial line O thereof.
Then, an impelling force and striking force toward the distal end side in the direction of the axial line O (the lower side in FIG. 6) and a rotating force around the axial line O are applied to the inner bit 102. Thereby, the inner bit 102 and the ring bit 103 engaging therewith excavate the ground to form a borehole while the casing pipe 101 is inserted (drawn) into the borehole.
Further, the inner bit 102 includes: a supply hole 104 which passes through the inner bit 102 and is open at the distal end portion of the inner bit 102; and a discharge groove 105 which is formed in the outer peripheral surface of the inner bit 102 and extends in the direction of the axial line O. Further, the supply hole 104 includes: a distal end blow hole 106 which is open in the distal end surface of the inner bit 102; and an outer peripheral blow hole 107 which is open in the outer peripheral surface of the inner bit 102. The distal end blow hole 106 is open into a distal end groove 108 which is formed in the distal end surface of the inner bit 102 and communicates with the discharge groove 105, and the outer peripheral blow hole 107 is open toward the distal end surface of the ring bit 103.
Then, during excavation, a fluid (an ejection medium) such as air is ejected onto the distal end surface of the inner bit 102 and the distal end surface of the ring bit 103 through the supply hole 104, while the fluid and a drill waste (a slime) generated by the excavation are discharged toward the tool base end side through the discharge groove 105.